Methionine sulfoxide (Met(O)) reduction is an essential metabolic pathway that provides protection against oxidative stress and regulates protein function. Met(O) are formed in the presence of reactive oxygen species and are reduced back to methionine by peptide Met(O) reductases. One Met(O) reductase (MsrA) has been known for decades and has recently been shown to regulate lifespan in animals. MsrA, however, is only specific for methionine-S-sulfoxides. The P.I. identified and characterized a second mammalian Met(O) reductase (SelR1) that is specific for methionine-R-sulfoxides. SelR1 is a selenocysteine-contain Jng protein and dietary selenium affects its expression. This raises a possibility that SelR1 may also be involved in delaying the aging process through reduction in levels of Met(O) and that supplementation of diet with selenium may provide means of extending the lifespan of certain segments of the human population. To directly characterize the role of SelR1 in aging, the pathway of Met(O) reduction in mammals will be analyzed with an emphasis on the function of selenoproteinSelR1 and characterization of the lifespan of animals that are either deficient or enriched in SelR1. A combination of biochemical and cell biology approaches and mouse model systems will be used to address the following specific questions (specific aims): 1) What are the properties and reaction mechanisms of SelR1 and its homologs? Three SelR isozymes have been identified in mammals. Wild-type and mutant forms of these proteins will be characterized and their catalytic activities, substrate specificity, metal-binding properties and the ability to complement yeast strains determined; 2) What are the tissue expression patterns, cellular locations and regulation of expression of Met(O) reductases? Hypotheses will be tested that SelR isozymes are located in different cellular compartments and that a single MsrA gene gives rise to two forms of the enzyme. In addition, efficiency of selenocysteine insertion and regulation of SelR1 expression by dietary selenium will be determined; 3) What is the role of SelR in aging? SelR1 knockout mice will generated and the hypothesis tested that these animals are characterized by a reduced lifespan. Transgenic mice overexpressing SelR1 will also be generated to determine whether these animals have increased lifespan.